[kotaKat]

This. I can just provision in the backdoor interface on the modem with a config file anyways and gain access.

Plus depending on model (like Arris modems), I can do things like set the password of the day seed (away from the factory default) to further lock it down and gain management access remotely.

[microtonal]

I don't know much about DOCSIS, but this is absolutely false for fiber here in The Netherlands. You can hook up your own OPNsense machine (or whatever you like) with an SFP+ module of your own choosing (as long as the transmitter is compatible, etc.). There is no way for the ISP to do any remote management.

DOCSIS is slowly dying here anyway and bleeding customers because the cable providers are not competitive when it comes to internet. If they didn't have better linear TV packages the bleed would even be larger.

[kotaKat]

DOCSIS modems require a configuration file from the cable modem termination system. This dictates the whole configuration from RF map and channel plan to things like QoS and management. Even with a customer-supplied modem, the CMTS will still supply a configuration file the modem must obey (and the CMTS will enforce).

For many modems on the customer market this also can mean that the ISP can push their own version of the firmware for a modem if you buy identical - such as pushing SURFboard updates.